Title THE ONSET OF SUMMER MONSOON OVERBANGLADESH

Author(s) HOQUE, Roxana

Citation第７回南アジアにおける自然環境と人間活動に関する研究集会 : インド亜大陸・インドシナの自然災害と人間活動 (2012)

Issue Date 2012-02-05

URL http://hdl.handle.net/2433/155858

Right

Type Presentation

Textversion author

Kyoto University

THE ONSET OF SUMMER MONSOON OVER BANGLADESH

The 7th Environment and Human Activity in South Asia‐Natural Disaster and Human Activity inthe Northeast Indian Subcontinent and Indochina” at Kyoto University. February 4-5, 2012

Roxana HOQUERoxana HOQUEDepartment of GeographyDepartment of Geography

Tokyo Metropolitan UniversityTokyo Metropolitan University

Geographic location of Bangladesh and world largest three majorriver’s the Ganges-Brahmaputra-Meghna

The term “monsoon” refers to the seasonal reversal of winddirection between winter and summer. Among monsoon phenomenaworldwide, one of the most fascinating is the South Asian summermonsoon, which causes a number of interesting changes inatmospheric circulation when it occurs over the Indian subcontinent(e.g., Matsumoto 1992).The monsoon seasonal transition from southwesterlies to easterliesis greatly affect Indian Peninsula, it leads heavy rainfall, flood andalso drought.The monsoon onset date is certainly crucial for tropical monsooncountry such as Bangladesh.Accurate prediction of the dates of monsoon onset and withdrawalwould assist farming enterprises in Bangladesh.

Introduction

Previous studies

Problem arose that broken isolines over Bangladesh indicates inadequate data !!

There is a clear gap remain over Bangladesh

Onset Withdrawal

They did not examine the monsoon seasonal transition and atmospheric circulation,such as moisture and precipitable water variables related those of the heavy rainfall aswell as onset has not been clear. They shows wind vector around Cox,s Bazar, but theydid not systematically examined wind direction. They also neglected pre-monsoon andpost-monsoon period, while pre-monsoon rainfall is interesting rainfall phenomena.

Ahm ed and Karm aker (1993: IJC )

O nset of rainy season

W ithdrawal of rainy season

The onset of rainy season in earlyApril in Assam region northeast India.2nd earliest onset found IndochinaLate April. The withdrawal foundIndochina Peninsula in late October.

(Matsumoto, 1997)

(Matsumoto, 1997: Advance in Atmospheric Science)Pentad mean OLR and wind at 850 hPa (April-June)

(Matsumoto, 1997: Advance in Atmospheric Science)Pentad mean OLR and wind at 850 hPa (Sept.-Nov.)

(Wang andLinHo,2002.J. Climate

Wang and LinHo (2002) defined the onset and withdrawal pentads of the rainy season in theAsian-Pacific monsoon domain. It is noteworthy result that the earliest onset of rainy season isfound in late April (P23-P24) over a limited area in the southeast BoB.

CMAP

BOE 100[ 120[ 140[

Date

160E

62 68 66 87 60 61 60 60

RA Fi7 Fi~M F.7 fi7 R7 fl7

180

53 62

66

Objectives

To obtain a better insight into behavior of the South Asian summermonsoon, it is crucial to understand the monsoon seasonaltransition and the seasonal march of monsoon rainfall from aclimatological perspective.

Large-scale circulation changes associated with monsoon seasonal transition, including the onset and withdrawal date.

Regional characteristics of seasonal rainfall variations in Bangladesh are described using the climatological pentad mean rainfall at each station and regional differences in each season.

Atmospheric circulation data used from Japanese 25-yr reanalysisdata (JRA-25) from 1979 to 2003.20-day mean wind at 850 hPa, moisture flux, precipitable water JRA-25 data provided by Japan Meteorological Agency (JMA), spatialresolution of 1.25° by 1.25°, with a time interval is 6 hours (Onogi et al. 2007).

Daily rainfall data at 35 stations for 61 years from 1948 to 2008,provided by the Bangladesh Metrological Department (BMD).

The 20-day mean data were calculated from the pentad data.

Data and Methods

Results 1Time series of twenty-day mean horizontal wind at 850 hPa,

water vapor flux, precipitable water, rainfalland

The definition of the monsoon onset and the withdrawal

The climetological pentad mean time series of the horizontal wind, the water vapor flux the precipitable water and rainfallaround (21.25 -25.0°N, 88.75 -92.5°E) from 1979-2003 over Bangladesh. The vertical dashes line in P31-32 and in P56-57indicates the onset and the withdrawal dates of summer monsoon in Bangladesh.

Time series provide newinsight into the seasonaltransitions and onset andwithdrawal In P31-32, all atmosphericvariables and rainfall greatlyincreased, is defined as themonsoon onset.In P56-57, all atmosphericvariables and rainfall sharplydecreased, is defined as the monsoon withdrawal.

Key feature of this studyThe definition of the monsoon

onset and the withdrawal

Results 2Atmospheric circulation

Distribution of twenty-day mean horizontal wind

(P32-35)-(P28-31)

Difference in horizontal wind at 850hPa frompre-monsoon to monsoon during onset period

P28-31 (May 16 - June 4) - P32-35 (June 5 - 24)

Onset

In P28-31 the sub-tropical westerly is located from west to east over northern India and Bangladesh. The strongnorthwesterly wind zone is also observed over India and Bangladesh, this wind is blows from the Himalayan region.In P32-35, the strong southwesterly wind dominated over the BoB and their direction continuously toward Bangladesh.

Intensification of southerly wind in the BoB, Bangladesh andIndia. Weak easterly or southerly change in wind direction.

P(57-60)-(53-56)

Difference in horizontal wind during monsoonand post-monsoon, withdrawal periods

P53-56 (Sep 18 – Oct 7) – P57-60 (Oct 8 – Oct 27)

Withdrawal

Not real anti-cyclone, but anti-cyclonic changeis occurring around BoB, India and Bangladesh

In P53-56, northwesterly wind and southwesterly wind flow anti-cyclonically turns toward the BoB andBangladesh. In P57-60, the northwesterly wind and easterly wind flow anti-cyclonically turns toward westernIndia.

Results 3Atmospheric circulation

Distribution of twenty-day mean precipitable water

P28-31

P32-35

(P32-35)-(P28-31)

Difference in Precipitable water frompre-monsoon to monsoon

P28-31 (May 16 - June 4) - P32-35 (June 5 - 24)

Many contour line makes little change, and little changemakes large difference, it is due to large gradient ofPrecipitable water (greater than 25 Kg m-2) . It is alsostrong influence of vapor and wind transportation.

In P32-35 and 28-31, changes in precipitable water may be due to the wind system, from pre-monsoon tomonsoon, and water vapor content from the foot Hill of Himalayan Mountain.

Onset

P57-60

P53-56 Difference in Precipitable water frommonsoon to post-monsoon

P53-56 (Sep 18 – Oct 7) – P57-60 (Oct 8 – Oct 27)

Comparison with onset, decrease in precipitable water observedin larger part of Bangladesh (-20), decrease may be migratefrom north to south, wind also decrease in this season

In P53-56, it is shown that the precipitable water still has a relatively high value about 45-50Kg m-2. In P57-60, precipitable water values suddenly decrease in the interior of Bangladesh tobe about 30-45 Kg m-2 .

Withdrawal

Results 4

Distribution of twenty-day mean rainfall

P28-31

P32-35

Two significant rainfall zones and two types of rainfall systems were identified. This changereflected to the increase in rainfall around the northeast, the southeast and the northwest regions.

Difference in rainfall from pre-monsoon to monsoonP28-31 (May 16 - June 4) - P32-35 (June 5 - 24)

(P32-35)-(28-31)Onset periods

P53-56

P57-60

Difference in rainfall from monsoon to post-monsoonP53-56 (Sep 18 – Oct 7) – P57-60 (Oct 8 – Oct 27)

A large difference also occurs between P57-60 and P53-56. In P53-56, rainfall peaksare located around northeast, northwest and the Bay region, decrease in rainfall

P(57-60)-(53-56)Withdrawal

Region W is relatively less rainfall area. Region S is also a comparatively heavy rainfall area.Region SE is one of the most significant rainfall zone around southern part. The abruptlyincreases in precipitation at Sylhet station, could not be combined with other station.

SE

S

W

Sylhet

The regional division of rainfall stations based on seasonal march of pentad meanprecipitation (35 rainfall stations divided into 4 groups).

0

50

100

150

200

250

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73

Pentad numberP

reci

pit

atio

n (

mm

)

Sylhet W SE S

Cluster analysis indicates seasonal march ofpentad means precipitation at Sylhet station isvery unique

Comparison with Ahmed and Karmaker (1993), this studydemonstrated a comprehensive definition of the monsoon seasons for thefirst time in Bangladesh. Result showed that the onset and withdrawal ofthe summer monsoon season in Bangladesh occurred between P31 andP32 and between P56 and P57, respectively.An abrupt change occurs between P31 and P32, at this time allatmospheric variables and rainfall are greatly increasing. Remarkablechanges also occur in P56-57, at this time, southwesterlies wind andwater vapor flux vanished not only in and around Bangladesh, but alsowhole Indian subcontinent. It has revealed that, V-wind component is a good indicator for definethe monsoon onset and withdrawal. Two significant rainfall zones andtwo different types of rainfall systems were identified. Cluster analysisindicates seasonal march of pentad mean precipitation at Sylhet station isvery unique.

Discussion and conclusions

This study presented mean conditions. Further study is needed toreveal the interannual variations and long-term changes in monsoonseasonal transitions in Bangladesh. Also it is important to investigatethe year to year rainfall variation.

The relationship between atmospheric circulation, ENSO and SSTpattern over the BoB region has not been fully investigated.

Future research

Time series of the climatological pentad mean 850 hPa U and V-wind (m/s)

Time series of the climatological pentad mean (green line) and 20-day mean (blueline) 850 hPa V-wind (a) and U-wind (b) field (m/s) from 1979-2003 over Bangladesh(21.25 -25.0°N, 88.75 -92.5°E). The abrupt increase in V-wind velocity is foundbetween in P31 and P33, which is in good accordance with the monsoon onsets,significant decrease between P56 and P57, which is in good accordance with themonsoon withdrawal over Bangladesh

(a) (b)

N S W WE

The climatological Precipitable water in (21.25 -25.0°N, 88.75 -92.5°E) from 1979-2003 over Bangladesh. Time series of the climatological pentad mean (green line) and20-day mean (blue line)

Time series of the climatological pentad mean Precipitable water

Time series of the climatological pentad mean (green line) and 20-day mean (blueline) of precipitation. Compared with the horizontal wind, the moisture flux, and theprecipitable water with time series of the pentad mean precipitation, it seems thechanges in rainfall are rather gradual. It is difficult to define the monsoon onset andwithdrawal date only by the times series of pentad mean precipitation.

Time series of the climatological pentad mean rainfall from 1948 to 2008

0

10

20

30

40

50

60

70

80

90

100

1 5 9 13 17 21 25 29 33 37 41 45 49 53 57 61 65 69 73

Pentad number

Pre

cipi

tati

on (

mm

)

Pentad mean 20-day mean